This invention relates to the field of corrosion protection systems and method and in particular for determining the true polarization potential in the system.
Use of cathodic protection systems to protect a cathode metal in contact with an electrolyte fluid is well known. Generally, cathodic protection systems are of two types--sacrificial anode or impressed current systems.
The sacrificial anode system relies upon the natural difference in electrical potential between a cathode and an anode to sacrifice or consume the anode to protect the cathode. As such systems inherently rely upon the natural difference in potential there is no need to measure and compensate for changes in the electrical potential between the anode and cathode.
The latter type system--impressed current--is considered more reliable, and usually relies upon a rectifier means to supply an impressed electrical direct current between the anode and cathode, but other sources of direct current may be used. For examples of such other power sources, see Control of Pipeline Corrosion by A. W. Peabody, copyrighted 1967 by, and available from, the National Association of Corrosion Engineers, 2400 West Loop South, Houston, Tex. 77027.
Generally speaking, the direct current impressed current producing rectifiers are powered by either 3-phase or single-phase alternating current (hereinafter AC) that is usually reduced in voltage by a transformer before being rectified into a direct current (hereinafter DC) output of a desired type. Normally, electrical current rectification is done by either a selenium or silicon rectifying disc or diode element which is electrically connected with similar discs to attain the DC voltage output desired. The known output is a positive direct current voltage and amperage having a rippling wave form of some function such as full wave, of the alternating current input to the rectifier.
Impressed current systems may also be used in an anodic passivation system such as disclosed in Bank, et al, U.S. Pat. Nos. 3,378,472; 3,375,183; and 3,371,023.
Precise control of the impressed current in a cathodic protection system is not only highly desirable, but has become a prime requirement. Early impressed current cathodic protection systems, for instance that disclosed in U.S. Pat. No. 2,176,514, lacked means for adjusting the impressed current to a changing environment. If the impressed current used was less than that required by the system, the system was inadequate and the undesired corrosion resulted. If, on the other hand, the impressed current used exceeded the system requirements electrical power was wasted and, equally important, paint "blistering" or other damage to the cathode's protective coating would result.
Earlier attempts to solve those problems used precise electrical output apparatus such as that disclosed in U.S. Pat. Nos. 2,332,955; 2,584,816; and 2,368,264.
However, it was quickly recognized that the cathodic protection system reference or natural voltage varied in response to a number of unrelated factors, such as temperature and condition of the protective coating on the cathode, and which factors that were changing from time to time. To compensate for such changes the rectifier output of direct current was made adjustable. Some were manually adjustable as disclosed in Polis U.S. Pat. No. 2,021,519. Early examples of automatically adjusting cathodic protection systems are disclosed in U.S. Pat. Nos. 1,891,005; 2,759,887; and 3,143,670, while U.S. Pat. Nos. 1,142,858 and 1,438,946 disclose general purpose output self-adjusting electrical apparatus. This automatic control or adjustment has usually been achieved in the prior art using saturable reactor control or with a silicon controlled rectifier (hereinafter SCR). For additional information, see the August 1968 article at pages 26-29 of Materials Protection, available from the National Association of Corrosion Engineers at the above address. Generally, the output of direct current from said sources have been pulsating which has created an error indicating output in the reference voltage.
In U.S. Pat. Nos. 2,986,512, 2,982,714; 2,987,461; and 2,998,371, all to Sabins, there is disclosed a number of control systems for automatically controlling the impressed current rectifier output. Another example employing solid state transistors may be found in Andersen, et al, U.S. Pat. No. 3,953,742 or Rubelman U.S. Pat. No. 3,373,100.
U.S. Pat. No. 3,129,154 discloses a compensating method of controlling the impressed current in which a known electromotive force is made opposed to the unknown reference electromotive force. In such arrangement, the electrical current flow through the reference circuit is minimized and polarization of the reference electrode, as well as the resulting deterioration, is minimized. The continuous pulsing anode current flow through the reference electrode created a voltage due to the IR drop which tended to induce an error in the reference voltage determination.
U.S. Pat. No. 3,634,222 disclosed an improved cathodic protection automatic control system in which the true cathode polarization potential could be determined using the "instant off" method. In this patent, the apparent reference potential is said to consist of the true reference potential and three error components that are removed by turning the system off. By turning the protection system off, the IR voltage drop resulting from the impressed current flow was eliminated and the true reference voltage drop determined. While such system does eliminate some of the error in determining the true potential, the system employed a sequential controller that was subject to failure and periodically interrupted the operation of the cathodic protection of the system and thereby leaving the cathode unprotected.
The related application identified above discloses a system that is synchronized to determine the natural reference voltage during zero voltage periods between pulses of impressed current in the electrolyte. In three-phase alternating current input system such zero output periods do not exist and that system is not usable.
The preciseness of any automatic control system is dependent upon the quality of the reference voltage signal received for indicating the system condition. This is true whether or not the reference voltage signal is used in an automatic controller or is simply indicated.